Resistor Sizes Vary Greatly?

[Wil_Bloodworth]

Hello!

I purchased some components from DigiKey.  Some are surprisingly small versus others that have the same power rating; 1W in this case.

How can the resistors in the middle, which are clearly listed as 1W resistors on the bag, handle the same amount of power as the resistors on the left and the right? 

Is it just because these larger resistors have a much higher resistance or am I missing something else?



Thank you,

- Wil

[wraper]

Small resistor is of compact type that withstands very high temperature to allow high power dissipation at small size. As of large ones I dunno why you bought them at all. They are high voltage resistors rated for 10kV.

[Wil_Bloodworth]

Yeah, I didn't know what I was ordering.

- Wil

[Zero999]

More specifically, the middle resistor is made from a ceramic material, which is why it can withstand high temperatures.

Note that, the middle resistor is probably rated to a much higher temperature, than most PCB materials and indeed other components. If it's mounted too close to the PCB, or an electrolytic capacitor, it will burn them. Either de-rate it, or use long leads and ceramic stand-offs to ensure it's a safe distance from the PCB and other components, so it doens't desolder iself, or burn anything.

[Wil_Bloodworth]

Thanks. I don't work with anything that has a PCB in it so I don't think that will be a problem (for now).

- Wil

[Siwastaja]

This discussion reappears every now and then, and some people have hard time believing the differences are real. But yeah, basically dissipating same amount of heat in smaller area must mean the surface temperature is higher. This must mean that the materials used by the manufacturer must handle this higher temperature.

There are other factors too, for example surface emissivity could be different, heat generation could be more even throughout the whole resistor so less hotspotting and thus better use of the available size without exceeding the material limit somewhere, but it's hard to say how significant these factors are in resistor design.

[TimFox]

In my youth (say, before 1970) in the US, most general purpose resistors were Allen-Bradley carbon composition (or competitors), and we all learned the package sizes for 1/4, 1/2, 1, and 2 W (and the rare 1/8 W).
Those resistors are now rare, but lead-bending gauges have kept the package dimensions as choices.
As discussed above, there are many technical details in the manufacturer's datasheets to guide the designer's choice of actual power rating for his circuit.
An interesting example:  the military rating on an RN65 metal-film resistor (0.562 inch body length) is 1/2 W.
The commercially-rated similar Vishay resistor CMF65 (same body size) is 1 W, but the datasheet has cautions about resistance changes caused by high power operation.
The older A-B 1 W carbon resistor is also 0.562 inch body length.
When in doubt, unless you are really cramped for space, derating the power on a resistor is a good idea.

[radiolistener]

it's hard to understand actual size from photo, but it looks that small one is 0.125 W.

0.25 W resistors a little bit bolder.

Here is comparison of size for different power:

[Wil_Bloodworth]

That is the point of the post... that small resistor is actually a 1W resistor (or at least it is listed as such) just like the two larger ones.

- Wil

[radiolistener]

that small resistor is actually a 1W resistor

this is Chinese 1 W 

1 W resistor cannot be the same size as 0.125 W, just because it dissipate 1 W power and needs large enough surface area to radiate it, so it cannot be small. If it has not enough surface area to dissipate heat it will be overheated, melted or damage other components around it.

[Wil_Bloodworth]

These resistors came from, as you can see, DigiKey so they are "reputable"... and they are saying they are 1W resistors... which is the entire point of me posting here. 

Your quote, "1 W resistor cannot be the same size as 0.125 W"... is the entire point of my question.

- Wil

[Siwastaja]

As always, just ignore radiolistener. He's probably on everybody's ignore list here.

Normal design cautions apply, as with all datasheet ratings; gotta understand what the datasheet parameters exactly mean. Thermal design is non-trivial and that applies even to older resistors with smaller power density. The better understanding you have, the closer you can calculate; you can always add fudge factors (i.e. derating) to hide uncertainties (parameters you don't know, lack of your understanding, incapability of purchasing expensive thermal modelling software etc.)

Heat damage done to surroundings is a very real issue even with larger resistors, it can't be ignored, and small components of course invite you to miniaturize your design, too, increasing the power density of the whole, which is the risky part, not using the small resistor itself.

[wraper]

it's hard to understand actual size from photo, but it looks that small one is 0.125 W.

0.25 W resistors a little bit bolder.

Here is comparison of size for different power:

FYI Soviet resistors came in different sizes for the same power rating.

[floobydust]

It's getting worse - there are many "mini" or "supermini" resistors being offered by resistor manufacturers nowadays that are typically 1/2 to 1/4 the size of what I am used to.

Last night I went to buy "1/4W" through-hole resistors for breadboarding, the 6-7mm wide parts I've used for decades.
I found "Metal Film Resistors - Through Hole .4watt 1% 1/8watt body size"   That is the 1/8W body size MBA0204 rated "standard mode" (0.25W) part max. is 125°C, "power mode" (0.4W) is 155°C.
Going upsize to the MBB0207 "standard mode" (0.4W) part max. is 125°C, "power mode" (0.6W) is 155°C. It's the size I'm expecting. At 1/4W it's temp rise is 35°C and old school would run it at 1/8W for temp rise 15°C.

Part of the new game and what has changed is:
-Higher temp materials (but who cares, nobody runs them that hot anyway) giving inflated power dissipation numbers that must compete with other spec stretchers.
-Thinner/thicker leads, and copper verses steel construction can give misleading graphs
-Specs that are for forced-air cooling use, in a fan-cooled (i.e. power supply) enclosure

So my old "1/4W" parts are rated 0.6W nowadays.
Most manufacturers are not giving thermal resistance or temperature rise data for their parts.
I commend Vishay for doing this, and curse Yageo for their game of omitting the spec and claiming a 1/4W sized part is "1W" FFS.

I did get a response from Vishay for this thread about the test jig used for the temp rise test. They (i.e. "0.4W") are the absolute max surface temperature (say 155°C) in 70°C ambient in the jig, 56 day run?. But I did not get it all figured out yet because many tests/specs are "common knowledge" in the resistor industry but never taught, to withstand the flames from people.
Dave could do a better investigation to help with the traps in choosing resistors.
SMT resistors are other whole set of traps, I just read how 0603's have a lower thermal resistance than 0805's due to their aspect ratio. Who'd have thought?

[coppice]

Small resistor is of compact type that withstands very high temperature to allow high power dissipation at small size. As of large ones I dunno why you bought them at all. They are high voltage resistors rated for 10kV.

Well you need about 2kV to get 1W out of a 3.9M resistor. 

[wraper]

Small resistor is of compact type that withstands very high temperature to allow high power dissipation at small size. As of large ones I dunno why you bought them at all. They are high voltage resistors rated for 10kV.

Well you need about 2kV to get 1W out of a 3.9M resistor. 

10KV is still a pulse voltage rating even if RMS voltage is limited by resistance. Any resistor series have max voltage rating even if it's actually must be lower for particular resistance to be within max heat dissipation.

[Gyro]

Call me cynical, but that middle resistor looks like a bog standard 1/4W 5% carbon film resistor that DigiKey have screwed up the data entry on.

[wraper]

Call me cynical, but that middle resistor looks like a bog standard 1/4W 5% carbon film resistor that DigiKey have screwed up the data entry on.

It neither looks 1/4W, nor carbon film. Also if you actually check the datasheet, yes it's this small.

[radiolistener]

These resistors came from, as you can see, DigiKey so they are "reputable"... and they are saying they are 1W resistors...

Two of them are VISHAY, yes this is reputable.

But the rest one which we're talking about is marked as "Manufacturer: YAGEO"

Are you sure that this is reputable manufacturer? 

[coppice]

These resistors came from, as you can see, DigiKey so they are "reputable"... and they are saying they are 1W resistors...

Two of them are VISHAY, yes this is reputable.

But the rest one which we're talking about is marked as "Manufacturer: YAGEO"

Are you sure that this is reputable manufacturer? 

YAGEO are the third largest passives maker in the world.

[wraper]

Are you sure that this is reputable manufacturer? 

I'm pretty sure that Yageo is the largest passive component manufacturer in the world.

[wraper]

YAGEO are the third largest passives maker in the world.

I guess you took it from Wikipedia. From what I've seen they are third in MLCC and inductors, first in resistors and tantalum capacitors, quite large at electrolytic and some other components. Overall at least by a total number of components produced should first too.

[coppice]

YAGEO are the third largest passives maker in the world.

I guess you took it from Wikipedia. From what I've seen they are third in MLCC and inductors, first in resistors and tantalum capacitors, quite large at electrolytic and some other components. Overall at least by a total number of components produced should first too.

Yeah, I knew they were huge, but I got that figure of third place from Wikipedia. 10 years ago it looked like Vishay would completely dominate passives, and simple semiconductors like small diodes. Now its looking like, through internal growth and acquisitions like KEMET, YAGEO is king.

[floobydust]

Yageo should be called scam-eo. The only reason they are selling well is noobs believing their fake specs - they don't give thermal resistance. When you withhold specs, same thing as lying.
OP's FMP series a usually 6.3mm 1/4w sized part- is rated 1W?! yet the usual 155°C max. rating.

Anyone care to explain how they get the 1W I'm all ears. The MFR-25 same size but 1/4W rated P₇₀.

It's total bullshit, where is the magical unicorn cooling fan?

[wraper]

As I see it, it's ambient temperature spec, not actual resistor temperature.